<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			/*
			外部尺寸是30px
			*/
			.a{
				margin-top: -20px;
				margin-bottom: 50px;
			}
		</style>
	</head>
	<body>
		<div class="a"></div>
	</body>
</html>
